The present invention relates to radio frequency module parts including surface acoustic wave elements of flip chip type which are mounted on a ceramic multi-layer substrate. In particular, this invention relates to radio frequency module parts including surface acoustic wave elements having increased reliability, improved element mounting efficiency, and reduced product size. The present invention also provides a manufacturing method for radio frequency module parts that enables increased productivity.
Miniaturization of electronic appliances is always increasing in the market. The size and weight of parts must also be reduced. These requirements are particularly remarkable in radio frequency appliances, such as mobile telephones. With respect to radio frequency appliances, electronic parts are mounted with high density, and the size and weight of the parts has been reduced. For miniaturization, a multi-layer substrate having plural conductor layers is mainly used in place of a conventional substrate having a single-layer conductor.
The ceramic multi-layer substrate is composed of an insulating layer formed of ceramics as an electrical insulator, and a conductive layer is formed with a metal, such as silver. In comparison to a general resin multi-layer substrate, the ceramic multi-layer substrate achieves low loss at radio frequency, good heat conductivity, dimensional high precision, and excellent reliability.
The ceramic multi-layer substrate is provided with internal conductors which are coiled or arranged opposite to each other in parallel so that inductance and capacitance may be formed therein. It is possible to form internal elements with low loss and high dimensional precision within the ceramic multi-layer substrate.
These ceramic multi-layer substrates are used as a module in radio frequency circuits such as mobile telephones. These ceramic multi-layer substrates are useful because various parts may be mounted on the surface to form small integrated elements.
Since radio frequency modules have circuits assembled based on each function, in comparison with a conventional technique of mounting discrete components one by one to form circuits, the structure of the module is simple and reliable. The present invention also enables modularization, which reduces manufacturing costs and simplifies device design.
FIG. 9 shows a block diagram of the radio frequency circuit of a mobile telephone of GSM dual band type which is most commonly used in the world. In FIG. 9, ANT is an antenna for transmitting and receiving electric waves, DPX is a diplexer (2-frequency filter) for separating a plurality of frequencies, T/R SW is a transmission/receiving switch for switching transmission and receiving, LPF is a low-pass filter for controlling the radio frequency at a transmission stage, and BPF is a band-pass filter at a receiving stage.
In such circuits of mobile telephones, the module is formed with a certain number of functions by actually mounting elements on the multi-layer substrate, for example, at the power amplifier portion in the transmission based circuit and at the antenna switch portion. FIGS. 10 and 11 show the structures of the respective examples.
FIG. 10 shows an example of a power amplifier module, where reference numeral 1 designates a dielectric multi-layer substrate having an internal electrode 1a and external electrodes 1b. On the dielectric multi-layer substrate, there are mounted MMIC which is a main portion of power amplifier and chip members 2 of the peripheral circuits. The MMCI is protected by a protection coating 3 and the upper side of the dielectric multi-layer substrate is entirely covered with a shield case 4.
FIG. 11 shows an example of a front-end module including the antenna switch portion, where reference numeral 10 is a ceramic multi-layer substrate having an internal inductance portion 11, capacitor portions 12, and external electrodes 13. Further, on the ceramic multi-layer substrate 10, there are mounted chip members 15 including diodes, resistors, and the like, and a shield case 16 for entirely covering the upper side of the ceramic multi-layer substrate. However, the front-end module of FIG. 11 does not include the surface acoustic wave element (hereafter the xe2x80x9cSAW elementxe2x80x9d).
Nowadays, module formation is realized in a single function such as a power amplifier and antenna switch module. Greater benefits of modularization may be obtained by including more functions in each module. It is a matter of course that the module formation including the SAW elements will be important.
The conventional SAW elements have been package members. It is possible to form the module by mounting the package members. However, as will be described in connection with the invention, mounting of element chips directly on the substrate enables a product which is miniaturized and reduced in thickness. Further, the production cost is reduced.
The ceramic multi-layer substrate, by having internal conductance and capacitance, achieves miniaturization. However, height reduction is more difficult. Therefore, the generally available module having the package mounted on the substrate will not satisfy the ever-increasing demand for reduction of height. Further, the package member occupies a wide area as compared to an original bear chip. Among the components to be used, the SAW element is one of the tallest components and occupies a broad area. Under these circumstances, the SAW element should be directly mounted to the ceramic multi-layer substrate without using the package.
On the other hand, for production of the SAW element, there is a process of forming the SAW chip and a process of mounting the SAW element to the package and sealing the same. Reduced costs for these processes are needed. If the SAW element may be directly mounted to the ceramic multi-layer substrate, the production cost will be reduced because the processes for mounting on the package and sealing may not be needed.
As has been described, regarding the radio frequency module, it is desirable to directly mount the SAW element to the ceramic multi-layer substrate to which other members are mounted by soldering.
In achieving the aforementioned goals, several problems arise, particularly:
(1) It is required to airtightly seal the SAW element chip;
(2) the structure should be resistant to temperature change without affecting the surface acoustic wave properties;
(3) the soldering process and the mounting process of the SAW element should be compatible.
(4) the module should have a flat surface while minimizing the module height;
(5) a plurality of ceramic multi-layer substrates should be mounted simultaneously, thereby increasing the production efficiency.
(1) Regarding the Requirement for Airtight-sealing the SAW Element:
The SAW element is made, for example, by forming a ladder-like electrode of aluminum at a precision of sub-micron (xcexcm) order, for example, on a lithium tantalite substrate. The electrode pattern is precisely designed for obtaining important properties including resonance frequency, bandwidth, insertion loss, and out-of-band loss. For example, an error of 1 xcexcm will not satisfy the design specification.
This precisely designed element is very likely to be influenced by external air. Contaminants such as water content from humidity and adherence of dust and the like damage the SAW element.
Accordingly, the SAW element mounting procedure should account for the above-described difficulties and should be compatible with miniaturization.
(2) Regarding the Requirement for Supporting the Element Free of Influence to the Surface Acoustic Waves:
When mounting the bear chips of a silicon based integrated circuit, the chips may be mounted to the substrate firmly and with entire surface adhered by, for example, an adhesive. However in the case of the SAW element, since the resonance property is obtained by the presence of elastic waves at the surface of the element, it is impossible to firmly fix the entire surface of the chips to the substrate by use of an adhesive.
In the case of the existing small SAW element, as disclosed for example, in JP-A-10-79638, the element is fixed to a ceramic substrate or a resin substrate by way of flip chip mounting. This is shown in FIG. 12, where reference numeral 20 is a substrate, and 30 is a flip chip as the SAW element. On the substrate 20, there is formed an electrode 21, the surface of which is gold (Au), and the flip chip 30 has a gold stud bump 31 formed on the main surface thereof which is formed with a ladder-type electrode for the SAW. The flip chip 30 is mounted (face-down-bonding) by way of gold-to-gold bonding where the main surface formed with the ladder type electrode for the SAW turned down.
It is also effective to follow this method for mounting the SAW element. However, the mounting process of the present invention is complicated when combined with other soldering members. Particularly in a composite module, the ceramic multi-layer substrate becomes thick. In this case, stress applied to the bonding portions is large compared to a normal package member.
(3) Regarding the Requirement for Making Compatible the Soldering Process and the SAW Element Mounting Process:
Generally in the soldering process, a solder paste is printed to the land portions on the substrate surface, and then the elements are placed and heat-treated through a reflow furnace fix the same thereon. In this case, a flux existing in the paste is evaporated to activate an interface relative to the surface electrodes, thereby securing wetness of the solder.
In the case of the present invention, the SAW element is mounted while exposed. If mounted in advance without securing air-tightness, flux may affect the properties of the SAW element.
Further, the SAW element is generally bonded by gold-to-gold bump bonding. In the case of the solder bonding, the metal surface on the substrate is a tin- film or a solder-film, and these are normally formed by plating.
(4) Regarding the Requirement for Flattening The Module Surface and Reducing the Height Thereof:
For mounting the electronic members, a method using an automatic mounting machine has been established and broadly employed. The apparatus normally includes a vacuum absorbing nozzle for handling the members. The member should therefore have a flat surface which is wider than at least the diameter of the nozzle. According to the prior art method, the surface of the composite module is covered with a metal plate. However, in the present invention, the addition of a flattening structure to the airtight structure is contrary to the requirement for reducing the height.
(5) Regarding the Requirement for Increasing the Production Efficiency by Treating a Plurality of Ceramic Multi-layer Substrates Simultaneously:
Normally, the ceramic multi-layer substrates are individually treated through a individual processes. However, the individual treatment requires much labor and decreases efficiency, which, in turn, increases production cost. Thus, it is desired to treat a plurality of pieces of the ceramic multi-layer substrates simultaneously.
JP-A-6-97315 discloses a preceding example of mounting the SAW element together with other circuit members and sealing the same. According to the preceding example, the SAW element is fixed as is turned up to a resin substrate and is electrically connected by wire bonding. This is different from the present invention, which mounts the SAW element to the multi-layer ceramic substrate in the form of a flip chip. According to the invention, by way of the flip chip mounting, miniaturization may be realized. By using the flip chip technique, the influence due to differences in the heat expansion rates between the chip and the substrate may be reduced. JP-A-6-97315 refers to the differences in the heat expansion rate between the ceramic substrates, which gives rise to problems. However, in the present invention, such influence is extremely little. Particularly, the temperature coefficient of the SAW element and the difference in the heat expansion rate are likely offset, and the temperature property of center frequency of the flip chip in the case of the resin substrate and in the case of the ceramic substrate is better in the ceramic substrate as shown in FIG. 4.
JP-A-6-97315 seems to disclose the mounting in combination with other passive members, but does not disclose the mounting together with the solder mounting members. Particularly, the solder is used for sealing but in this case, for avoiding contamination by flux, an instantaneous heating method is employed. In short, it is suggested that the mounting in combination with the solder mounting members is extremely difficult. In the present invention, the mounting together with other solder mounting members may be available by taking a cleaning process (cleaning process); and the present invention simplifies the problem and enables other various members to be mounted together.
Accordingly, in view of the above-mentioned difficulties, it is a first object of the invention to provide radio frequency module parts including SAW elements and a manufacturing method thereof, enabling the mounting of a SAW element as a bear chip together with other solder mounted members.
It is a second object of the invention to provide radio frequency module parts including SAW elements and a manufacturing method thereof, where the SAW element may be mounted as a bear chip, thereby to secure miniaturization, reduction in height, increase of production efficiency, and reduced manufacturing costs.
Other objects and characteristics of the invention will be apparent in the following description in reference to the modes for carrying out the invention.
For attaining the above-noted objects, the radio frequency module parts including SAW elements according to a first aspect of the invention are characterized in that a ceramic multi-layer substrate is mounted thereon with SAW elements and other surface mounting elements, the surface acoustic wave elements being flip chips which are face-down-bonded by way of gold-to-gold bonding to gold-coated electrodes of the ceramic multi-layer substrate, and at least the SAW elements being covered airtight with side walls fixed to the ceramic multi-layer substrate and a cover covering the opening of the side walls.
According to a second aspect of the invention, the radio frequency module parts including SAW elements as set forth in the first aspect of the invention is characterized in that at least one of the other surface mounting elements is mounted on the ceramic multi-layer substrate by way of soldering.
According to a third aspect of the invention, the radio frequency module parts including surface acoustic wave elements as set forth in the first or second aspect of the invention is characterized in that said surface acoustic wave elements are isolated from said other surface mounting elements by the side walls and the cover.
According to a fourth aspect of the invention, the radio frequency module parts including SAW elements as set forth in the first, second, or third aspect of the invention are characterized in that the area of a portion surrounded by the side walls and covered by the cover is between approximately 30% and 100% of said ceramic multi-layer substrate.
According to a fifth aspect of the invention, the radio frequency module parts including SAW elements as set forth in the first, second, third, or fourth aspect of the invention are characterized in that the gold film of the gold plated electrodes of the ceramic multi-layer substrate is between about 1 xcexcm and about 5 xcexcm, the space between the SAW elements and the gold plated electrodes is between about 5 xcexcm and about 50 xcexcm, and the diameter after bonding the gold bumps for the gold-to-gold bonding is between about 50 xcexcm and about 150 xcexcm.
A method of making radio frequency module parts including SAW elements according to a sixth aspect of the invention includes the steps of: plating a gold plate to at least portions of bonding parts of conductive surfaces of a ceramic multi-layer substrate; mounting by soldering, after the plating procedure, at least one surface mounting element other than the SAW elements to the ceramic multi-layer substrate; cleaning the multi-layer ceramic substrate after the soldering; mounting, after the cleaning procedure, flip chips of the SAW elements via face-down-bonding to the ceramic multi-layer substrate by gold-to-gold bonding; bonding members becoming side walls to the ceramic multi-layer substrate; and airtight sealing a space surrounded by the members becoming the side walls and a member becoming a cover by attaching the member becoming the cover after mounting the SAW elements and forming the side walls.
According to a seventh aspect of the invention, the method of making the radio frequency module parts including the SAW elements as set forth in the sixth aspect of the invention is characterized in that the side wall forming procedure is carried out by bonding the side wall forming members in common to plural pieces of the ceramic multi-layer substrates, and at least parts of the following steps thereafter are carried out simultaneously, and the members finally becoming the sides are cut per each of ceramic multi-layer substrates.
According to an eighth aspect of the invention, the method of making the radio frequency module parts including the SAW elements as set forth in the sixth or seventh aspect of the invention is characterized in that the cleaning procedure is for cleaning said ceramic multi-layer substrate by way of a plasma etching.